(1) Field of the Invention
This invention relates to a developing device employed in a color copier or the like.
(2) Description of the Related Art
A conventional developing device of this type has been disclosed in Japanese Patent Publication Kokai No. 63-261282. According to this invention, a plurality of developing units are provided around a photoconductive drum, and each developing unit is driven by driving means provided at an end of the photoconductive drum. Such a construction has three main problems: 1) the construction of a driving system is inevitably complicated; 2) the complicated construction around the photoconductive drum and the strictly-restricted arrangement prevent compactness and efficient maintenance; and 3) since each developing unit has different positional relationship with the photoconductive drum, it is hard to uniformize shapes and constructions of the developing units.
The inventors of the present invention have disclosed another conventional developing device in Japanese Patent Application No. 63-83502. According to this invention, a developing device comprising a plurality of developing units arranged in shelves from top to bottom is moved up and down by a developing device driving motor and each developing unit is driven by a corresponding developing unit driving motor for development. However, this invention has the following problems since the developing units, each of which has the developing unit driving motor, increases the weight and size of the whole developing device: 1) the large force of inertia for moving and stopping the developing device deteriorates controllability of moving up and down the device; 2) the large load applied on the developing device driving motor requires the motor to be large-sized, thereby to enlarge the size of the whole device; 3) the large-sized device increases manufacturing cost; 4) the large-sized device is required to move a long distance and takes a long time for selecting one of the developing units, which prolongs operation time; and 5) since the size of each developing unit is determined by the specification torque of the corresponding developing unit driving motor, the design of each developing unit and the whole device is restricted.